Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago and resembles a phonograph. That is, the hard drive model includes a storage disk or hard disk that spins at a standard rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The arm carries a head assembly that has a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The complete head assembly, e.g., the suspension and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable hard drives such as those used in personal digital assistants (PDAs), MP3 players, and the like. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters of less than 2.5 inches (micro drives are significantly smaller than that). Advances in magnetic recording are also primary reasons for the reduction in size.
A second refinement to the hard disk drive is the increased efficiency and reduced size of the spindle motor spinning the disk. That is, as technology has reduced motor size and power draw for small motors, the mechanical portion of the hard disk drive can be reduced and additional revolutions per minute (RPM) can be achieved. For example, it is not uncommon for a hard disk drive to reach speeds of 15,000 RPM. This second refinement provides weight and size reductions to the hard disk drive and increases the linear density of information per track. Increased rates of revolution also provide a faster read and write rate for the disk and decrease the latency, or time required for a data area to become located beneath a head, thereby providing increased speed for accessing data. The increase in data acquisition speed due to the increased RPM of the disk drive and the more efficient read/write head portion provide modern computers with hard disk speed and storage capabilities that are continually increasing.
However, such high rates of revolution of the disk have produced a greater need for accurate balancing of the disk pack, in a manner analogous to the need to balance the wheels of an automobile.
In general, unbalance during rotation occurs when the physical axis about which the body is rotating (e.g., the mounted rotational axis such as the central drive hub) and the mass axis of the rotating body (e.g., the mathematical axis of balance or the inertial axis) are not aligned. Normally, the non-alignment of the rotational axis and the inertial axis occurs in three forms. That is, as shown in Prior Art FIG. 1, the rotational axis 160 and the inertial axis (e.g., axis 172) may be parallel to each other, the rotational axis 160 and the inertial axis (e.g., axis 171) may intersect each other, or the rotational axis 160 and the inertial axis (e.g., axis 173) may not be parallel or intersect each other.
When the rotational axis 160 and the inertial axis (e.g., axis 172) are parallel to each other this is known as static or single-plane unbalance. As apparent from Prior Art FIG. 1, the magnitude of the force created by the static imbalance is equal across the length of the spinning component (e.g., from the top to the bottom). Moreover, the direction of the force vectors is also constant across the component.
When the rotational axis 160 and the inertial axis (e.g., axis 171) intersect each other this is known as a couple unbalance. As apparent from Prior Art FIG. 1, the magnitude of the force created by the couple imbalance are equal at both ends of the spinning component (e.g., the top and the bottom), but the force vectors while equal in magnitude on both ends of the component are opposite in direction.
When the rotational axis 160 and the inertial axis (e.g., axis 173) may not be parallel or intersect each other this is known as dynamic or coupled-plane unbalance. As apparent from Prior Art FIG. 1, the magnitude of the force created by the dynamic imbalance is not equal across the length of the spinning component (e.g., differs from the top to the bottom) and the force vectors on both ends may differ substantially. For example, the top imbalance plane 185 has an imbalance force vector 180, shown as Fa, pointing at an angle which is different from the angle of the imbalance force vector 190 from the bottom imbalance plane 195, shown as Fb. As is apparent, the chances of the imbalance force vector Fa 180 randomly aligning with the imbalance force vector Fb 190 is extremely minimal.
Without proper balance, a spinning disk pack can vibrate undesirably. Such vibrations can have numerous deleterious effects. For example, disk vibration can change a relative position between a head and a disk. Such detrimental changes in head positioning can result in less reliable read/write performance of a hard disk drive, including, for example, track misalignment, an inability to read a desired track and/or deleteriously overwriting an adjacent track. In addition, disk vibration can result in the production of undesired sound energy. For example, unwanted disk vibration can produce an undesirable sound energy in the disk drive enclosure and/or in a disk drive mounting system. Such sound energy can produce unwanted audio noise in a computer system, e.g., a desktop computer system, leading to an unacceptable experience for a computer user.
Further, disk vibration can cause other deleterious effects. For example, vibrations from one hard disk drive can be mechanically coupled to other hard disk drives in a system, leading to a variety of ill effects across many drives within a drive mounting system.
Disk vibration has at least one other highly undesirable consequence related to overall disk reliability. Modern electronic systems, e.g., computer electronics, are highly reliable. Moving parts of computer systems, e.g., fans and hard disk drives, are generally the least reliable components of such systems. As a consequence, a great deal of engineering effort has been invested in making such components more reliable in a quest to make the overall system more reliable. In addition to other deleterious effects, disk vibration induced by pack imbalance(s) leads to increased wear and hence lessened reliability of a spindle motor and its bearings within a hard disk drive. For example, a hard disk drive with low frequency vibration will tend to wear out, or fail, sooner than a similar hard disk drive without such deleterious vibrations. Consequently, drive vibration induced by pack imbalance(s) lead to undesirably decreased system reliability.
Conventionally, two masses are utilized to balance a disk drive comprising multiple disks. A first mass is placed on top of a disk and/or disk-spindle motor assembly, and a second mass is placed on the bottom of the disk and/or disk-spindle motor assembly, generally offset in angle with respect to the first mass. The proper positioning and selection of these two masses can correct a static and dynamic imbalance of a disk and/or disk-spindle motor assembly.
However, just as the need for better balancing of disk packs has increased due to ever increasing disk revolution rates, the overall reduction in drive and component size coupled with decreased “empty” space within a hard disk drive, has made it more difficult to balance such disks. For example, as the overall height, or thickness, of a hard disk drive decreases, and/or a number of platters increases, there is less space available for the addition of balancing masses to a disk or stack of disks. Additionally, manufacturing processes utilized to assemble small, highly dense hard disk drives have difficulty accommodating additional process steps that may be required to add balancing masses to disks.
Further, as modern heads “fly” extremely close to a disk, the environment within a hard disk drive must be kept very clean. For example, hard disk drives are typically manufactured in class-100 clean rooms, and incorporate filters to clean the air inside of a hard disk drive. The introduction of balance masses may introduce undesirable contaminants to the head disk enclosure, either via the masses themselves and/or via additional manufacturing process steps required to add the masses to the drive. Additionally, inclusion of additional parts, e.g., balancing masses, deleteriously requires additional process steps to clean such parts, incurring further undesirable manufacturing costs.
Yet another drawback to the use of balance masses is the direct cost of such balance masses. If utilized, such balance masses should be produced to very strict engineering specifications, e.g., for material, diameter, shape, density etc. As such, the unit cost for such balance masses can be both unexpectedly and undesirably high. It is appreciated that the manufacturing process steps required to place such balance masses further incur process costs, e.g., process time and/or additional manufacturing equipment. Consequently, it would be advantageous to eliminate at least one such balance mass.
Accordingly, there is a need for apparatus and methods for correcting static and dynamic imbalance with a single mass in a hard disk drive. Additionally, in conjunction with the aforementioned need, methods and systems for correcting both static and dynamic imbalances in a disk stack are desired. A further need, in conjunction with the aforementioned needs, is for balancing disk assemblies in a manner that is compatible and complimentary with existing hard disk systems and manufacturing processes.